Conventionally, for improving quietness of a brushless motor, a control method for smoothing switching of energization by sine-wave drive so as to reduce a torque ripple is known. In the sine-wave drive, a current having a current value changing in a sinusoidal waveform is supplied to a motor whose induced voltage has a sinusoidal waveform. As a result, the sine-wave driven motor has a small torque ripple as compared with a general 120-degree square-wave driven motor. Moreover, as a method of reducing the torque ripple, a skew structure for inclining rotor magnetic poles or the like with respect to an axial direction is widely known. In a motor using the skew structure, harmonic components of the induced voltage waveform are reduced. Thus, the induced voltage waveform may be changed into a smooth sinusoidal waveform. Accordingly, the effects of reducing the torque ripple at the time of the sine-wave drive are further improved.
In the motor having the skew structure, however, a fundamental component of the induced voltage is reduced with the reduction of the harmonic components if the skew angle is increased. The magnitude of a torque entirely depends on the fundamental component because the output torque of the motor is approximately equal to a product of the induced voltage and the motor current. Therefore, if the skew angle is increased to reduce the torque ripple, the motor torque is reduced to lower a voltage-use efficiency. On the other hand, if motor specifications are set with the small skew angle to suppress the reduction in induced voltage so as to ensure the motor torque, there is a problem in the increased torque ripple because the harmonic components are correspondingly increased.
Therefore, in recent years, for improving the voltage-use efficiency and reducing the torque ripple at the same time, a control method for causing the motor driving current to contain the harmonic components so as to cancel the harmonic components of the induced voltage has been proposed. According to the control method, the fundamental component is ensured with the small skew angle, while the induced voltage is changed into the sinusoidal waveform by the harmonic components of the motor-driving current. As a result, the increased output of the motor is realized while the torque ripple is reduced.    Patent document 1: JP 2006-174692 A
In the control method for causing the motor-driving current to contain the harmonic components as described above, however, it is necessary to set a control cycle according to the harmonic components. Therefore, in a control mode in which a large number of high-order harmonic components having a short wavelength are contained, there is a problem in that the control cycle is required to be set extremely short. As a result, the control is extremely difficult. In addition, there are problems in that a load is imposed on, and cost becomes high for, a control system correspondingly.
An object of the present invention is to provide a brushless motor realizing a low torque ripple and a high output at the same time, which may be easily controlled.